Belts for automobiles, such as timing belts, poly ribbed belts, wrapped belts and V-belts, are composed of a complex of reinforcing cords and matrix rubber (a rubber-fiber complex). Conventionally, chloroprene rubber (CR) and acrylonitrile-butadiene copolymer rubber (NBR), which are oil resistant rubber, have been mainly used as matrix rubber. However, to cope with an increasing demand for heat resistance with the need to comply with emission regulations for automobiles, reduce the size of engine rooms for reducing the weight of automobiles, insulate engine compartments for better noise control, and so on, such nitrile group-containing highly saturated copolymer rubber that balances heat resistance and oil resistance has been commercially available.
Generally, a reinforcing cord is formed by a band of reinforcing fibers, each having a coating on the surface thereof for protecting the surface of the reinforcing fiber and increasing adhesiveness to the matrix rubber. To form this coating, a mixture of a resorcin-formaldehyde condensate and latex (which may be referred to hereinafter as “RFL”) is commonly used. For example, JP 63-270877 A (PTL 1) discloses a reinforcing cord with increased adhesiveness to the matrix rubber. In this reinforcing cord, a coating is formed on the surface of each reinforcing fiber constituting the reinforcing cord. The coating is obtained by applying, drying and curing a mixture of a resorcin-formaldehyde condensate and a latex of hydrogenated nitrile rubber (H-NBR) on the surface of reinforcing fiber.
However, a timing belt using this reinforcing cord with coatings, which are formed by applying, drying and curing a mixture of a resorcin-formaldehyde condensate and a latex of hydrogenated nitrile rubber (H-NBR), has poor oil resistance at the boundaries between the reinforcing cord and the matrix rubber, i.e., at the coatings on the reinforcing fibers constituting the reinforcing cord when used as an in-oil belt of an automobile, resulting in insufficient oil resistance of the in-oil belt.